Supercharged internal combustion engines may be operated in different modes based on engine output. For example, when an engine is operated at lower engine speed and/or load range, impulse supercharging may be employed to enhance the operating behavior of the engine. Impulse supercharging advantageously uses the dynamic wave produced during charge exchange for supercharging to enhance engine operations, which may produce pressure fluctuations on the turbine. Alternatively, at higher engine speeds, a more constant exhaust-gas pressure may be desired for smoother engine operations since the turbine operates with a higher efficiency when shocks and pressure fluctuations on the turbine are reduced. Thus, ram supercharging may be employed to reduce fluctuating partial loads on the turbine during periods of high engine output.
Evacuation of combustion gases out of an engine cylinder during charge exchange is based substantially on two types of mechanisms. When an outlet valve opens close to bottom dead center, charge exchange begins and combustion gases flow at a high speed through the outlet opening into the exhaust-gas discharge system on account of the high pressure level prevailing in the cylinder at the end of combustion. The rate of flow may be based on the associated pressure difference between the combustion chamber and an exhaust line associated therewith. Said pressure-driven flow process is assisted by a high pressure peak which is also referred to as a pre-outlet shock that propagates along the exhaust line at the speed of sound, the pressure being dissipated, or reduced, to a greater or lesser extent with increasing distance traveled as a result of frictional forces present. During the further course of charge exchange, the pressures in the cylinder and in the exhaust line are equalized, and combustion gases are no longer evacuated in a pressure-driven manner but instead are discharged as a result of the reciprocating movement of the piston of the combustion chamber.
At low loads or engine speeds where low exhaust-gas flow rates prevail, pre-outlet shock may be advantageously utilized for impulse supercharging, as a result of which it is possible to obtain high turbine pressure ratios even at low turbine rotational speeds. To utilize the dynamic wave phenomena occurring in the exhaust-gas discharge system, in particular the pre-outlet shocks, for impulse supercharging to enhance the operating behavior of the internal combustion engine, the pressure peaks or pre-outlet shocks in the exhaust-gas discharge system must be maintained. It is particularly advantageous if the pressure fluctuations are intensified in the exhaust lines. It is therefore expedient for the cylinders to be grouped, or for the exhaust lines to be merged, in such a manner that the high pressures, in particular the pre-outlet shocks of the individual cylinders, in the exhaust-gas discharge system are maintained during operation. However, turbine efficiency increases when shocks or fluctuating partial loads (e.g., pressure fluctuations) are reduced. Thus, at high engine speeds, a constant exhaust-gas pressure, for which reason a pressure with small fluctuations upstream of the turbine is employed in order to realize so-called ram supercharging. As a result of a correspondingly large exhaust-gas volume upstream of the turbine, pressure pulsations in the exhaust lines can be smoothed by grouping the cylinders into merged exhaust flows.
A conflict thus arises in exhaust-gas discharge systems with regard to optimal performance at low engine speeds and high engine speeds, or so as to optimize the exhaust-gas discharge system both with regard to low exhaust-gas flow rates and with regard to relatively high exhaust gas flow rates. Grouping the cylinders to realize impulse supercharging produces expedient operating behavior at low engine speeds, but degrades turbine efficiency at higher speeds due to pressure fluctuations. In contrast, if a large exhaust-gas volume is realized upstream of the turbine in order to be able to utilize the advantages of ram supercharging by increasing the volume of the exhaust lines to smooth the pressure fluctuations at relatively high engine speeds, the operating behavior at low engine speeds, the so-called low end torque, may be impaired.
Concepts are known in which the two exhaust manifolds of the two cylinder groups are connected to and separated from one another in a manner that depends on engine conditions. The exhaust-gas discharge system is then configured as a function of the engine speed or gas dynamics, such that supercharging of the internal combustion engine by impulse supercharging is realized by separating the exhaust manifolds whereas supercharging of the internal combustion engine by ram supercharging is realized by connecting the exhaust manifolds. However, disadvantages arise when connections are positioned close to the outlet openings of the cylinders, since the residual gas problems described above, and associated knocking problem, are abetted, that is to say intensified. Alternate systems may adjust a supercharging mode based on the position of a valve such as a poppet valve whose position can be either open or closed. However, such systems have the weakness that open/close states of the valve do not allow fine adjustment of the exhaust coupling across the range of operating conditions. Said differently, a difficulty arises since the extent of exhaust coupling between each of the two channels is not allowable, particularly in combination with exhaust gas blow-off, which may be advantageous under some conditions.
The inventors have recognized issues with such approaches and herein describe a two-channel turbine comprising a longitudinally displaceable flow transfer valve arranged within a flow transfer duct that couples a first channel of the two-channel turbine to a second channel, the valve arranged lateral relative to the flow of exhaust gas. Structural features of the flow transfer valve are further included that allow for controlling the extent of communication between the first and second channels, in addition to a waste gate passage based on a position of the valve relative to the flow transfer duct. In this way, the technical result is achieved that the flow transfer valve allows for adjusting a mode of supercharging based on the extent of fluidic coupling using a simplified valve arrangement and controls thereof.
In one particular example, positioning the flow transfer valve into a rest position within the duct completely separates the first and second channels of the two-channel turbine while simultaneously blocking exhaust airflow to the waste gate passage, the exhaust airflow through the first and second channels being directed separately to a rotor of the two-channel turbine. Then, adjusting the position of the flow transfer valve to a first position within the duct allows communication between only the first and second channels of the two-channel turbine while simultaneously blocking exhaust airflow to the waste gate passage, wherein the extent of communication between the first and second channels is determined responsive to the extent that a plunger-like end of the flow transfer valve occupies a first recess of the flow transfer duct. Further adjustment into a second position within the duct allows communication between both the first and second channels as well as the waste gate passage included therewith. Based on the structural features of the valve described in greater detail below, in the second position, the extent of communication between both the first and second channels and the waste gate passage is determined responsive to the position of the plunger-like end relative to a second recess of the flow transfer duct, which advantageously allows for fine adjustment of the exhaust blow-off. In other words, the system according to the present disclosure allows for the rate of exhaust-gas flow conducted past a rotor of the two-channel turbine via a blow-off line to be adjusted based on the position of the valve in the duct.
In an internal combustion engine according to the present disclosure, the volume of the exhaust-gas discharge system communicating with an individual channel of the turbine can be varied, specifically by virtue of the two channels of the turbine being connected or separated based on the position of the flow transfer valve relative to the flow transfer duct. Consequently, it is also possible for the exhaust-gas volume or the exhaust-gas discharge system upstream of the at least one rotor of the two-channel turbine to be adapted to different operating conditions of the internal combustion engine, in particular to different exhaust-gas flow rates or different engine speeds, and optimized in this regard.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.